1. Field of the Invention
The present invention relates to a rare earth metal-based permanent magnet and a process for producing the same, wherein the formation of a corrosion-resistant film such as a plated film can be carried out at a high dimensional accuracy.
2. Description of the Related Art
A rare earth metal-based permanent magnet such as an R—Fe—B based permanent magnet represented by an Nd—Fe—B based permanent magnet is produced using a material which is rich in resources and inexpensive and has a high magnetic characteristic, as compared with an Sm—Co based permanent magnet. Therefore, particularly, the R—Fe—B based permanent magnet is used in a variety of fields at present.
In recent years, in electronic industries and appliance industries, a reduction in size of parts is advancing, and in correspondence to this, a reduction in size and a complication in shape of the magnet itself are required.
From this viewpoint, the public attention is paid to a bonded magnet easily formed from a magnetic powder and a resinous binder as main components. Such a bonded magnet is already put into practice use in various fields. However, the rare earth metal-based permanent magnet contains a rare earth metal R which is liable to be corroded by oxidation in the atmosphere. Therefore, when the rare earth metal-based permanent magnet is used without being subjected to a surface treatment, the corrosion advances from the surface of the magnet under the influence of a small amount of an acid, an alkali or water to generate a rust in the magnet, thereby bringing about the deterioration and dispersion of the magnetic characteristic. Further, when the magnet having a rust generated therein is incorporated in a magnetic circuit, it is feared that the rust is scattered to pollute the surrounding parts.
To solve this problem, an attempt has been made to form a plated film as a corrosion-resistant film on the surface of the magnet. However, when the bonded magnet is subjected directly to an electroplating treatment, a uniform and dense plated film cannot be formed, because the magnetic powder particles insulated by the resinous binder forming the surface of the magnet and the resin portion between the magnetic powder particles are lower in electric conductivity. As a result, pinholes (non-plated portions) may be produced to bring about a rust in some cases.
With the above point in view, various processes have been proposed in which an electric conductivity is provided to the entire surface of the bonded magnet, and the bonded magnet is subjected to an electroplating treatment.
For example, Japanese Patent Application Laid-open No.5-302176 describes a process which involves placing a bonded magnet, a resin which is at least in a partially uncured state, an electrically conductive powder and a film forming medium such as steel balls into a vessel, where a resinous film including the conductive powder is formed on the surface of the magnet by vibrating the vessel or by agitating the contents of the vessel, and forming a plated film on the resulting surface.
Japanese Patent Application Laid-open No.7-161516 describes a process which involves forming an uncured resinous layer on the whole or a portion of the surface of a bonded magnet, then forming an electrically conductive layer of a metal powder on the surface of the resinous layer using copper balls which are media for a vibrated-type ball mill, and further forming a plated film on the surface of the conductive layer.
Japanese Patent Application Laid-open No.11-3811 describes a process which involves immersing a bonded magnet into a solution of a coupling agent containing a metal powder added thereto, thereby adhering the metal powder to the surface of the magnet, coating the metal powder onto the surface of the magnet in a filled manner by a striking force of blast media such as stainless balls, and then forming a plated film on the resulting surface.
Further, Japanese Patent Application Laid-open No.8-186016 describes a process which involves coating a mixture of a resin and an electrically conductive material powder onto the surface of a bonded magnet to form an electrically conductive film layer, subjecting the magnet to a surface smoothing treatment, and forming a plated film on the resulting surface.
The following processes have been proposed as a method for forming a corrosion-resistant film other than a plated film on the surface of a bonded magnet:
For example, Japanese Patent Application Laid-open No.7-302705 describes a process which involves coating the surface of a bonded magnet with an uncured resin, placing the resulting magnet into a vessel along with a metal powder and film forming media such as balls made of alumina, and adhering the metal powder onto the surface of the uncured resin by vibrating the vessel and/or by agitating the contents of the vessel, thereby forming a chromate film on the resulting surface.
Japanese Patent Application Laid-open No.10-226890 describes a process which involves immersing a bonded magnet into a solution of a coupling agent containing a metal powder added thereto, thereby previously depositing the metal powder onto the surface of the magnet, adhering the metal powder by blast media such as stainless balls, and forming a resinous film on the resulting surface.
Japanese Patent Application Laid-open No.9-205013 describes a process which involves filling a metal powder into the voids in the surface of a bonded magnet by an attacking force of blast media such as steel balls, and forming a resinous film on the resulting surface.
The processes described in Patent Application Laid-open No.5-302176, 7-161516, 11-3811 and 8-186016 basically provide an electrical conductivity to the entire surface of the bonded magnet, using the metal powder. Even by the processes described in Patent Application Laid-open Nos.7-302705 and 10-226890, an electrical conductivity can be provided to the entire surface of the bonded magnet. However, any of the processes is intended to adhere the metal powder onto the surface of the magnet by utilizing the stickiness of the third component such as the resin and the coupling agent. In such processes, an increase in cost is brought about, because the third component is required. In addition, it is difficult to form the electrically conductive layer uniformly on the entire surface of the magnet and as a result, it is difficult to achieve the surface treatment at a high dimensional accuracy. Additionally, a step of curing the uncured resin is required, resulting in a complicated producing process. Further, when media such as steel balls, copper balls, stainless balls or alumina balls are used as a metal powder adhering means, it is feared that cracking or chipping of the bonded magnet are brought about.
According to the process described in Patent Application Laid-open No.9-205013, the metal powder can be filled in the voids in the surface of the magnet without use of a third component such as a resin and a coupling agent. However, this process is not intended to adhere the metal powder on the magnetic powder forming the surface of the magnet. Therefore, even if the metal powder is adhered on the magnetic powder, the adhering force is necessarily weak and hence, it is impossible to adhere the metal powder onto the magnetic powder. In addition, a step of removing the surplus metal powder weakly adhered to the magnetic powder by washing is required in this process and hence, the complication of the producing process is brought about.